Method of coating electrodes



Nqv. 20, 1934,

G. E. LONVG HEAJIN'G I HYDROGEN PR EGLOWING ELECTRODES SPRAYING I HCOLLOIDAL GRAPHITE DRYING HEATING IN VACUUM INVENTOR 6. ELONG 9mm 644ml;

A T TORNE V I 45 of the grid to prevent emission therefrom.

Patented Nov. 20, 1 934 I UNITED STATES PATENT OFFICE METHOD OF COATINGELECTRODES George E. Long, Orange, N. .L, assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a. corporation of New YorkApplication April 28, 1931, Serial No. 533,577

3 Claims. (01. 250-275) This invention relates to a method of coatingconstant to dissipate the heat generated in the electrodes for use inconnection with electron device. emitting electrodes of electrondischarge devices. In accordance with this invention the elecin highpower electron discharge devices, partrodes, namely, the grid and theanode, are coat- 5 ticularly of the oscillator type, the use of high edby a method which forms a coalescent hard anode and grid voltagesresults in pronounced film of carbon on the grid or the anode. The

heating of the refractory metallic electrodes. carbon coating having ahigh black body con- Usually thecathode associated with the anode stantserves to dissipate the heat generated in and grid is provided with ahighly thermionic the electrodes thereby rendering these electrodesmaterial either combined with the 'core or uncooler and overcoming to agreat extent second- 65 combined in the form of a coating. Under suchary emission from the grid electrode. When temperature condition, thegrid or control elecparticles ofthermionic material are deposited on.trode becomes an emitter of electrons. When the the grid from thecathode, these particles are grid emits electrons, a phenomenon .knownas prevented from introducing primary emission efblocking effect occurs,thereby paralyzing the f'ects, due presumably to the formation of car itcontinued operation of the device. The grid hides which are notthermionically active and emission generally falls into two distinctclasses are sufiiciently stable at the operating temperaand may bedefined as primary emission, due to tures encountered in the device soas not to particles. of thermionic material being thrown aifect theoperation of the device. Furthermore, off the cathode and deposited onthe hot grid the carbon film, while in effect a high resistance .iiwherethey form a source of electrons, and seccoating, does not detract fromthe proper funcondary emission, due to the high temperature of tioningof the wire grid as a control electrode the grid and the velocity of theimpinging elecin the flow of electrons from the cathode to trons causingthe emission of secondary electrons the anode.

from the grid. Naturally, this adverse condition The drawing showsdiagrammatically the proc- 3) of the grid should be overcome if the gridis to ess steps required to obtain the results of this perform itsnormal function of controlling the invention.

flow of electrons from the cathode to the anode. The method of coatingthe electrode with 2,

However, when attempts are made to overcome -film of carbon consists inpre-glowing by heatemission from the grid by coating this electrode ingthe grid or anode in a vacuum or reducing at with a black substance tomaintain the grid atmosphere to completely free the electrode of coolerand also to combine with thermionic maoccluded gases and deleterioussubstances. The terial so as to form inactive compounds, the electrodeis then coated with a colloidal solution characteristics of the deviceare changed due to of g p k-IWI1 Commercially as Aquadag the differencein the diameter of the lateral wires' a d fi y t Carbon is fi t e ectode to of the grid and the spacing between adjacent surface in a vacuumor reducing atmosphere of wires. Furthermore, the thick black coatingshydrogene fi m Of carbon formed on the usually in the form of oxides ofthe metal of the Surface by s fix thod is relagrid practically insulatethe grid from the other tlvely hard and Smooth and firm1y electrodes andit does not assume its normal funcherem to surfzffie; Furthermore, thediamtion until the insulating properties of the coatgg i gg gfig g :5;gsggg ig igigfi i mg are broken down by heatmg Slmnafly some istics ofthe device in which his associated with metals, such as molybdenum, donot react fath d d d I bly when coated with an oxide of the metal a 6 anVora In accordance with this invention the anode 100 or grid is firstdeoxidized by heating the metallic 'lhe principal obiect of thisinvention is to body in a reducing gas Such as hydrogen, or in f mpnmary a emlsslon from the a vacuum to remove occluded gases and oxy engrid inelectron d1SC -r forming compounds which may deleteriously af-Anotlfler ObJeFt of the mve1 1t1n to jiorm feet the carbonizing process.This step produces 10 Protectwe coatlflg on the r whlch W111 P a cleanand bright surface on the electrode which aifect the electricalcharacteristics of the device. is then ready f the carbonizingtreatment; The

A 1 6 obl c 0 e Invention 15 0 form carbonizing of the wire grid or theanode conprotective coating on an electrode, such as an sists inspraying or otherwise coating the suranode, the coating having a highblack body face of the grid or anode with a'concentratedl I slightlyammoniacal solution of colloidal graphite commerciallyknown as Aquadag.The sprayed electrodes are allowed to dry and are then subjected to afixing" treatment in order to insure complete coalescence between thecarboncoating and the surface of the electrode. This fixing process maybe carried out by either of two methods of heat treatment.

In the first method the sprayed electrode is placed in a reducing ovenand heated to a temperature from 500 to 800 C. while hydrogen or otherreducing gas is injected into the oven. The

' flxing" process may also be carried out in a methods.

and fixing process high vacuum at a temperature from 600 to 900 C. Ineither fixing process the colloidal graphite coating. on the electrodeis formed into a closely adherent film on the electrode which film is,very diiiicult to remove even by abrasion A major advantage of thismethod is that while the coating is of sumcient thickness to accomplishthe desired result of considerably lowering primary and secondaryemission, it is thin enough that it does not in any manner change theelectrical characteristics of the devices. The

controlling of the dimensions of the grid struc-.

ture is a primary requisite in electron discharge technique since achange in diameter of the grid wire of 0.002 inch will materially affectthe dynamic and electrostatic characteristics of the device. For thisreason a large number of carbon producing compounds are objectionablesince they result in a relatively thick coating of caricon on thegridand therefore cause variations in the spacing between the wires of thegrid electrode.

trodes which are usually associated with low temperature coatedfilaments.

The pre-glow treatment of the electrodes in accordancewith thisinvention, prior to the carbonizing and flxing" treatments preventsalteration or the carbon film on the electrodes exposure to air prior toinserting the electrodes within an evacuated vessel. Furthermore, thepre-glowed carbonized electrode insures freedom from oxygen formingcompounds which may combine with thermionically active material underelectronic action of the cathode and deleteriously affect the cathodecoating material.

It is of course understood that other metals such as iron and alloymetals generally employed as electrodes in electron discharge devicesmay during the ,fixing treatment and subsequent be carbonized by theprocess as herein described.

Therefore, the invention is only to be limited within the scope ofthe'appended claims.

What is claimed is:

L The method of carboniaing an electrode for an electron dischargedevice which comprises preglowing the electrode to remove occludedgases, coating the electrode with a colloidal graphite solution, andheating the electrode at temperatures from 500 to 900 C.

2. The me hod oicarbonizing an electrode which comprises heatingtheelectrode in a reducing atmosphere ;.to remove occluded gases,

coating the electrode with colloidal graphite,

fixing the graphite coating in a reducing atmosphere, and heating theelectrode at a temperature from 500 to 800 C. while in the reducingatmosphere.

3. The method of carbonizing an electrode.

which comprises heating the electrode in a vacu-' um to remove occludedgases, spraying the electrode with a concentrated ammoniacal solution ofcolloidal graphite drying in air, and finally heating the electrode inan evacuated vessel within a range of temperatures from 600 to 900 C. tofix the carbon to the electrode surface.

.GEORGE E. LONGr- V use

